Canaries are small, fragile, sensitive creatures weighing no more than 20 grams, about seven-tenths of an ounce. They have become a familiar and significant metaphor, however, due to the important role they played as vivid warning indicators of substantial threats to human welfare. Because canaries are extremely sensitive to the presence of methane and carbon monoxide—deadly but odorless gases that seep from deep coal deposits—miners in England and the U.S. carried canaries in little cages along with them as they worked in underground coal seams. When the canaries began to sway and slump noticeably on their perches, the miners could react immediately, realizing that something was terribly wrong for humans as well.
The diminutive “snail darter”—the small endangered fish still remembered derisively as an icon of over-reaching wildlife law for its role three decades ago in obstructing completion of the TVA’s last water project, the Tellico Dam—likewise served in fact and function as a canary in a coal mine. The darter’s role as an indicator of large issues of human welfare and governance, however, never garnered the familiarity and acknowledgment achieved by the canary. The little fish was and is generally cloaked in a cloud of mocking dismissal, remembered as a small technicality misused by extreme environmentalists to block human progress.
Zygmunt J.B. Plater. "Classic Lessons from a Little Fish in a Pork Barrel - Featuring the Notorious Story of the Endangered Snail Darter and the TVA's Last Dam." Utah Environmental Law Review 32, no.2 (2012): 211-244.